Biomedical Engineering Reference
In-Depth Information
can be made, but most AFM instruments also limit the size of the sample that can fit
into the sample (sample-scanning instruments are particularly limited). Specific
instruments which allow scanning of very large samples do exist, however, they
will typically include automated sample/head movement to allow for scanning of
various areas across a large sample. Such instruments are typically aimed at industrial
applications, e.g. scanning of whole semiconductor wafers.
• The sample has to be rigidly mounted into the AFM sample stage. A sample that is not
well fixed down will tend to move while scanning, leading to distortion in the image.
Vibration of the sample can also add noise to the image. The most common sample
mounting for AFM is using a mounting disk made of magnetic stainless steel. This has
the sample glued to it, sometimes using epoxy adhesive, which is highly rigid once
cured, although double-sided adhesive tabs are also popular for less demanding
applications. The magnetic disk is placed in the sample holder, which has a magnet
in the centre. This arrangement keeps the sample very stable, and greatly reduces
sample movement and vibration. Alternative arrangements where it is undesirable to
use a magnet under the sample (e.g. for magnetic modes, or for optical access to the
sample from below), usually involve some sort of sprung clips to securely hold down
the sample.
Specific sample preparation techniques
The number of different types of samples that can be scanned by AFM precludes
describing each one here, but it is possible to give some tips on preparing some of the
most commonly examined samples here.
Particulate samples
Micro- and nanoparticles of all imaginable geometries and materials are very common
samples for AFM, and imaging of a very wide range of different particles has been widely
described [217, 278-284]. Often such samples come as an aqueous dispersion. The first
step is to ensure the sample is as clean as possible, especially if the particles are very
small (where the effect of contaminants is greater in relative terms). Where the dispersion
is known to be very concentrated it should be then diluted. Often the ideal image will
feature dispersed particles, so that the dimensions of the individual colloids can be
measured. If the sample is to be imaged in air, then the sample is simply deposited by
dropping a known volume onto a flat substrate and allowed to dry. Although AFM can
operate either in air or liquid environments, imaging a sample that still retains significant
amounts of water in air can be problematic, therefore improved imaging after drying
samples thoroughly is common [285]. Often drying small (
100 nm) particles onto a flat
surface is enough to 'fix' then adequately for AFM analysis, especially for examination
by either IC-AFM or NC-AFM. For contact mode, especially for larger particles, such a
procedure might not adhere the particles well enough to the surface. In this case, it might
be necessary to have some chemical fixing to the surface, or use a special substrate
(see below) [286].
Two factors in the sample preparation method that can have dramatic effects on the
quality of results obtained are the solvent used to disperse the particles, and the substrate
used. Water is generally the solvent of choice for such applications, as it is convenient,
<
